The invention relates to a steering force control apparatus for a power steering system and, in particular, to a reaction control valve unit which controls a hydraulic pressure acting upon a hydraulic reaction mechanism thereof.
A conventional steering force control apparatus for a power steering system is disclosed in Japanese Laid-Open Patent Application No. 68,467/1988 in which a torsion bar is twisted in accordance with an input applied to a steering wheel to switch a servo valve to introduce a hydraulic pressure into one of the chambers on one of the sides of a piston of a power cylinder, thereby imparting a steering assisting force to steerable road wheels. At the same time, to allow a driver of a vehicle to sense a resistance to a steering operation, there is provided a hydraulic reaction mechanism to develop a reaction or a force resulting from a steering operation which comprises a reaction to the twisting of the torsion bar, and a reaction component produced by a hydraulic reaction chamber which is acted upon by pressurized oil, as well as a reaction pressure control valve which controls the reaction to be introduced into the hydraulic reaction chamber.
In such a power steering system of the vehicle speed responsive type and utilizing a hydraulic reaction, the steering force generally comprises two components, namely, (1) a steering force component which results from a hydraulic reaction which is linearly proportional to the cylinder pressure, and (2) another component produced by the torsion bar in proportion to the square root of the cylinder pressure. The first component is generally controlled in a variable manner so as to correspond to the vehicle speed, by utilizing the reaction pressure control valve.
In the steering force control apparatus disclosed in Japanese Laid-Open Patent Application No. 68,467/1988 cited above, the steering force which is produced by the reaction of the hydraulic pressure exhibits a response which is linearly proportional to the cylinder pressure, so that the steering force increases in linear proportion to a load upon the power steering system under a high lateral G condition when running at high speeds, which is sometimes less desirable for the performance of the power steering system.